1. Field of the Invention
The present invention relates to an imaging apparatus, and more particularly, to a printhead mounting apparatus providing adjustment to effect printhead skew correction in an imaging apparatus, such as a laser printer.
2. Description of the Related Art
In a typical in-line color electrophotographic imaging process, latent images are formed on a plurality of photosensitive drums, which are in turn each developed using a predetermined color of toner. Typically, these colors are black, magenta, cyan and yellow. The developed images are then transferred to either an intermediate transfer medium or directly to a sheet of media (such as paper) that travels past the photosensitive drums. The image in each color is created one line at a time, and the lines are oriented at right angles to the direction of travel of the media. The individually-generated images combine to form a full-color image. Thus, in a typical multi-color laser printer, the sheet of media receives color images generated at each of the four image developing stations.
It is recognized that in order for the multi-color laser printer to print accurately, the laser beams for all four colors must be in alignment, both in the scan direction (i.e., the direction the laser sweeps across the photoreceptive medium) and the process direction (feed direction of the print medium). However, providing proper alignment of even a single laser printhead in relation to the sheet of media in the process direction can be difficult. This problem is compounded with the addition of each printhead, since the plurality of printheads must be in registration so that the individual images generated by each printhead can be superimposed correctly when combined. During printer assembly an attempt is made to optically align the laser printheads both individually and collectively, but the ability to provide precise alignment is limited by several factors, including component manufacturing tolerances. In addition, it is possible for a precisely aligned printhead to drift out of alignment over time due to component aging and ambient environmental factors, such as temperature. Skew is one such alignment parameter that can be corrected by mechanical rotation of the printhead relative to a pivot point located in the printer mounting frame. Skew is the slope of a least squares fit straight line through all of the laser spots across a scan line.
It is known that one can use one of a variety of sliding or pivoting mechanisms to mount a printhead to a printer frame and to provide adjustment of the position of a printhead in an adjustment direction to correct printhead skew. However, such sliding mechanisms are difficult to control when attempting to make small adjustments necessitating movement of the slide mechanism over a very short distance. In attempting small adjustments, there is a likelihood that the mechanism will exhibit the phenomenon know as xe2x80x9cstick-slipxe2x80x9d, or frictional hysteresis, which makes repeatability uncertain. For example, an executed command to move the slide mechanism a certain distance at one time may not be repeatable in producing the same amount of motion at another time. In addition, when the fasteners mounting the printhead to the mounting frame are tightened, unwanted rotation can produce an error in skew registration.
What is needed in the art is an apparatus that can consistently provide precise and repeatable printhead skew adjustment in an imaging apparatus to compensate for printhead alignment errors due to printhead skew.
The present invention provides an apparatus that can consistently provide precise and repeatable printhead skew adjustment in an imaging apparatus to compensate for printhead alignment errors due to printhead skew, and in addition, provides compensation for thermal dimensional variations of the printhead.
One aspect of the invention relates to an imaging apparatus including a machine frame unit having a plurality of mounting locations and a pivot location, a printhead base having a first end and a second end, and a first resilient elongate member having a first proximal end and a first distal end. The first proximal end is attached to the first end of the printhead base. A mounting tab is coupled to the first distal end of the first resilient elongate member. The first mounting tab is coupled to a first mounting location of the plurality of mounting locations of the machine frame unit. A mounting plate is coupled to the second end of the base. The mounting plate is coupled to at least a second mounting location of the plurality of mounting locations of the machine frame unit. A pivot post, having an axis of rotation, pivotally couples the mounting plate to the pivot location of the machine frame unit. An adjustment device is coupled to the machine frame unit, the adjustment device having an engagement member for engaging at least one rigid feature of the printhead base and the first resilient elongate member to effect a deflection of the first resilient elongate member and a corresponding movement of the printhead base. At least the first resilient elongate member is configured to allow for translation which in turn allows the printhead base to move with respect to a virtual pivot axis without significantly distorting optical mounting relationships within the housing when the mounting tab and the mounting plate are fixedly attached to the machine frame unit. A location of the virtual pivot axis substantially corresponds to a location of the axis of rotation of the pivot post.
In another aspect of the invention, a method is provided for correcting printhead skew in an apparatus having a machine frame unit to which a printhead having a printhead housing is mounted. The method includes the steps of: defining a desired scan path for the printhead; identifying an actual scan path of the printhead; determining an amount of skew between the desired scan path and the actual scan path; providing an eccentric shaft which engages the machine frame unit and the printhead housing rotating the eccentric shaft to effect an initial positioning of the printhead housing with respect to the machine frame unit; providing a plurality of fasteners to fixedly attach the printhead housing to the machine frame unit following the initial positioning; and providing a fine adjustment mechanism to effect a rotation of the printhead housing about a virtual pivot axis to provide a final positioning of the printhead after the printhead housing is fixedly attached to the machine frame unit.
One advantage of the present invention is that precise incremental changes in the orientation of the printhead can be easily effected.
Another advantage of the present invention is that it allows for thermal dimensional variations of the printhead, and in particular, the length of the printhead housing structure, without inducing a torque on the housing that would affect skew relationships.
Still another advantage is that the printhead orientation fine adjustment mechanism is not adversely affected by frictional hysteresis, or by tightening mounting fasteners.